1. Field of the Invention
The invention is generally related to LED devices. More particularly, the invention is related to the design of silicon sub-mount LED devices.
2. Related Art
Conventional LED (Light Emitting Diode) devices such as the LED device 100 pictured in FIG. 1 are encapsulated by a clear and/or translucent plastic with two protruding metal contacts. One of these contacts is called the anode while the other is the cathode. The back of the LED die is attached (using conductive solder) to one metal contact while a wire bond is used to complete the electrical circuit therein by attaching to the other metal contact. One disadvantage of this typical LED arrangement is that there is no built-in protection for Electro-Static Discharge (ESD) but is cheap to manufacture.
In recent vogue have been xe2x80x9csilicon sub-mountxe2x80x9d based LED arrangements. One such arrangement 200 is illustrated in FIG. 2. The sub-mount has a topside to which a vendor""s LED chip can be attached. The sub-mount has electrical connectors such as solder bumps which are exposed on its topside. These connectors electrically couple the LED chip""s anode and cathode. The sub-mount is placed upon the cathode. The sub-mount, on its underside, has one connection connected to the lead frame. This underside connection cannot be electrically insulated from the lead frame since there is a need to dissipate heat generated by the LED chip. The sub-mount is wire-bonded to the lead-frame by way of two wire bonds, one coupling to the anode, and the other coupling to the cathode. In this arrangement, the silicon sub-mount includes two back-to-back Zener diodes (well-known in the art) which provide ESD protection. The Zener diodes"" cathodes connect to the anode and cathode of the lead frame and the Zener diodes"" common anodes connect to the underside of the sub-mount. Again, the entire sub-mount and LED chip assembly along with the two wire bonds are encapsulated in a similar fashion to the more conventional LED described above.
The sub-mount LED arrangement 200 of FIG. 2 while providing ESD protection (by way of the Zener diodes) has several disadvantages. Unlike the more conventional LED arrangement shown in FIG. 1, the sub-mount LED requires two wire bonds. This adds to manufacturing costs and can adversely impact reliability. Further, if a voltage is applied in a reverse direction (i.e. reverse to that xe2x80x9cforward directionxe2x80x9d which would cause the LED to illuminate) there is only a 0.6 voltage drop due to the forward biasing of one of the Zener diodes. If a user connects the LED backwards (i.e. with the opposite polarity) to a low-impedance voltage source, the arrangement will consume a huge amount of current, heat-up and likely burn out.
Other arrangements include a two wire bond LED without silicon sub-mount. This arrangement would suffer from the disadvantage of having an extra wire bond as well as the disadvantage of not having any ESD protection.
There is a need thus for a silicon sub-mount solution that can avoid these difficulties while still featuring built-in ESD protection.
What is disclosed is an apparatus consisting of a single wire bond silicon sub-mount used to make an LED device and which also has built-in ESD protection in the sub-mount. The single wire bond silicon sub-mount uses a pass-through interconnection between the topside of the sub-mount and the underside so that the LED chip mounted thereon is electrically coupled through the sub-mount to the anode. The single wire bond is used to couple the LED chip via the topside of the sub-mount and through the sub-mount all the way to the cathode. The pass-through interconnection provided by the silicon sub-mount eliminates the need for a second wire bond and enables a single wire bond connection between the lead frame (anode and cathode) and the LED chip. The ESD protection circuitry is by way of two back-to-back series connected Zener diodes.